Pump bearing arrangement

ABSTRACT

A pump bearing arrangement, suitable particularly for rapidly rotating pumps such as turbomolecular pumps, comprises a pump rotor ( 10 ) con-nected to a rotor shaft. The rotor shaft ( 14 ) is supported in a shaft housing ( 20 ) by two bearing arrangements ( 12 ). Each bearing arrangement ( 12 ) has an inner bearing ring ( 16 ) connected to the rotor shaft ( 14 ) and an outer bearing ring ( 22 ) connected to the shaft housing ( 20 ). Bearing supports ( 24 ) are disposed between the two bearing rings. At least one vibration element ( 26 ) is provided between the outer bearing ring ( 22 ) and the shaft housing ( 20 ), serving for damping and further having a thermal conductivity of at least 0.3 W/mK for dissipating heat.

The invention relates to a pump bearing arrangement which is suitable particularly for rapidly rotating pumps such as e.g. turbomolecular pumps.

Pumps, such as e.g. turbomolecular pumps, comprise a pump rotor connected to a rotor shaft. The rotor shaft is supported in a shaft housing via two bearing arrangements. Said two bearing arrangements usually comprise respectively one ball bearing. The inner bearing rings and respectively bearing shells of the two ball bearings are fixedly connected to the rotor shaft. The outer bearing rings and respectively bearing shells are connected to the shaft housing in the axial direction, or are arranged in abutment e.g. on an annular projection of the shaft housing in the axial direction. The two bearing arrangements are axially biased. In the radial direction, the outer bearing rings are usually not arranged in direct abutment on the shaft housing. Instead, an annular gap is provided between the outer bearing rings and the shaft housing. In a circular groove formed in the shaft housing and surrounding the outer bearing ring, an O-ring is arranged, serving particularly as a vibration element for damping and radially holding each of the two bearing arrangements. As vibration elements, use is made e.g. of O-rings manufactured from silicone. This, however, gives rise to the problem that the heat dissipation effected via silicone O-rings will be merely poor.

For improving the heat dissipation, it is known from DE 8 910 040 to fill said vibration element, such as e.g. the O-ring, with metal powder and preferably copper powder. Particularly those O-rings which comprise metal and respectively copper contents, however, will tend to change their mechanical properties, which is due to the compression of the copper elements occurring during operation. This may affect the damping properties and the position of the bearing arrangements. Admixture of metal or copper powder to an elastomer further entails the risk that the distribution of the particles will not be homogeneous. This will give rise to regions with increased particle density, so-called nests, as well as regions with reduced particle density. As a result, unpredictable local variations of thermal conductivity will take place within the O-ring. As a result, the elastic properties of the vibration element will deteriorate. Especially in regions of increased particle density, the stiffness and respectively elasticity of the vibration element will be caused to vary. Pump bearing arrangements, particularly those of fast-rotating pumps such as e.g. turbomolecular pumps, are subjected to permanent vibration. Thereby, vibrations of up to 1.5 kHz may occur. This can lead to a decomposition or segregation of the metal and respectively copper particles in the O-ring. This effect will lead to internal friction of solids and a resultant considerable increase of temperature in the vibration element. In case of such a decomposition or segregation of the particles on the surface of the vibration element, particles may happen to escape from the vibration element and penetrate into the bearing. Such occurrences will reduce the operating life of the bearing. Said decomposition or segregation will further cause changes of the vibration properties as well as local thermal over-stressing of the material. This may lead to complete destruction or plastic deformation of the vibration element.

Particularly in rotors of pump bearing arrangements for use in fast-rotating units such as turbomolecular pumps, the problem exists that the friction generated in the bearings will cause a massive heat-up of the bearing arrangements. The temperatures generated in such fast-rotating components cannot be sufficiently dissipated by known vibration elements such as O-rings made of silicone or O-rings including copper contents. This is the case particularly in pump bearing arrangements for fast-rotating turbomolecular pumps which typically are designed for a rotor speed of 500 Hz-1.500 Hz, depending on the rotor diameter.

Heat dissipation from the bearing is difficult to achieve particularly because the axial surfaces of the ball bearings used, which preferably are angular-contact ball bearings, are small and thus will contribute only little to heat dissipation. In the radial direction, the outer bearing rings are usually arranged at a distance from the shaft housing for allowing the axial bias to be realized. Consequently, heat dissipation in the radial direction will not be sufficient. Particularly since the bearings are operated at low pressures of usually 1×10⁻³ to 10⁻¹⁰ mbar, heat dissipation in the radial direction is extremely poor.

It is an object of the invention to provide a pump bearing arrangement, particularly of the type suited for rapidly rotating pumps, wherein the heat dissipation is improved.

According to the invention, the above object is achieved by the features defined in claim 1.

As provided by the invention, the vibration element serving for damping is designed and/or arranged to have a thermal conductivity of at least 0.3 W/mK. The vibration element is located between the bearing ring and the shaft housing. Preferably, the vibration element surrounds the outer bearing ring and consequently is arranged between the outer bearing ring and the shaft housing. According to the invention, the vibration element thus fulfills two functions, i.e. on the one hand, damping the movements of the bearing ring and, on the other hand, dissipating the heat from the bearing ring particularly into the shaft housing. This inventive dual function of the vibration element makes it possible to realize a good heat dissipation from the bearing. Thereby, the operating life of the bearing is extended.

With particular preference, said at least one vibration element has a thermal conductivity of at least 0.35 W/mK and more preferably at least 0.4 W/mK. Preferably, the vibration element comprises an elastomeric material or is made of elastomeric material. Tests have shown that the material Elastosil R840 is especially well-suited for use in the realization of the invention.

With particular preference, use is made of a quasi-homogeneous material for the at least one vibration element. In this case, the inventive thermal conductivity of the material is guaranteed not by admixture of particles such as e.g. metal or copper powder but, instead, by the elastomeric material itself. Herein, the properties of the material are influenced particularly by the length and the structure of the macromolecules. In this regard, it is preferred to select a material in which the vibration and heat-conductivity properties are substantially identical in all spatial directions and which, particularly, is not susceptible to material decomposition even in case of large temperature influences and temperature variations, especially in the range of 4° C. to 120° C. Even when pumps, particularly fast-rotating pumps such as turbomolecular pumps, are subjected to high vibration stresses up to 1.5 kHz, said material which is preferred according to the invention will still remain elastic. Particularly within the frequencies normally occurring in this application field, especially in the range from 1 Hz to 4 kHz, the material is permanently vibration-proof so that no changes of the material properties will occur.

Preferably, the material to be used comprises a silicone rubber mass.

Particularly, a plural number of vibration elements can be provided for each bearing arrangement. Preferably, two annular vibration elements are provided, surrounding the outer bearing ring so as to take up radial forces. Further, for axial damping, vibration elements—preferably again of an annular shape—can be provided also in the axial direction.

According to a preferred embodiment, the vibration elements, which preferably are of a rectangular or circular cross-sectional shape, are respectively arranged in a groove. Preferably, said groove is arranged in the shaft housing, with the vibration element preferably extending beyond the groove so that there will always be a gap formed between the shaft housing and the bearing ring.

The vibration element preferably has a Shore A hardness of 40-80 Shore A.

The invention further relates to the use of a vibration element having a thermal conductivity of at least 0.3 W/mK in pump bearing arrangements. Herein, the vibration element is preferably designed in the above described manner.

The invention will be explained in greater detail hereunder by way of a preferred embodiment with reference to the accompanying drawing.

The FIGURE shows a schematic lateral view of an upper and respectively rotor-side bearing arrangement according to the invention.

A preferred embodiment of the bearing arrangement 12 arranged in an upper position, i.e. facing toward a rotor 10, is illustrated in the FIGURE. Rotor 10 is connected to a rotor shaft 14 for common rotation therewith. Arranged on rotor shaft 14 for common rotation therewith is an inner bearing shell 16 of a grooved ball bearing, which is held in position by the rotor and the shaft stub 18.

The upper bearing arrangement 12 is arranged in a shaft housing 20 which in the illustrated embodiment has a substantially cylindrical shape. An outer bearing ring 22 is arranged internally of shaft housing 20. Between said two bearing rings 16,22, bearing bodies in the form of balls are arranged.

In the first embodiment, outer bearing ring 22 is connected to shaft housing 20 by two radial vibration elements 26 formed as elastic O-rings. Further provided is an axial vibration element 28, preferably also formed as an elastic O-ring. Said two radial vibration elements 26 are respectively arranged in an edge region of outer bearing ring 22 in a respective groove 30. Said groove 30 is provided in shaft housing 20. The depth of groove 30 is smaller than the diameter of said O-rings 26 so that the latter projects beyond an inner side 32 of shaft housing 20 and are located in abutment on outer bearing ring 22.

The axial vibration element 28 is arranged in an axial annular groove 34 which also is provided in shaft housing 20. Also here, the depth of groove 34 is smaller than the diameter of O-ring 28 so that the latter projects in the direction of outer bearing ring 22. Between axial vibration element 28 and outer bearing ring 22, a force transmission element 36 is provided. Said force transmission element 36 particularly has the function of a friction damper. A further function of force transmission element 36 resides in allowing for movement of outer bearing ring 22 in the radial direction without causing shear forces or radial forces to act on axial vibration element 28. At least, the corresponding forces that occur can be significantly reduced.

Between the two radial vibration elements 26, a venting bore 38 formed as a transverse bore can be provided.

The annular vibration elements 26,28 provided in the illustrated embodiment serve for heat dissipation from the bearing arrangement 12. The heat, generated especially by the rolling friction of the balls 24, is thus dissipated via outer bearing ring 22 as well as via vibration elements 26,28 into shaft housing 20. For this purpose, vibration elements 26,28 have a good thermal conductivity, preferably higher than 0.3 W/mK.

In addition to the illustrated upper bearing arrangement 12, a further ball bearing is provided to support the lower end of rotor shaft 14. Said two bearings are tensioned for providing an axial bias in the longitudinal direction of rotor shaft 14. 

1. A pump bearing arrangement, particularly for rapidly rotating pumps such e.g. as turbomolecular pumps, comprising a rotor shaft connected to a pump rotor, and a shaft housing with the rotor shaft supported therein via two bearing arrangements, each bearing arrangement comprising an inner bearing ring, an outer bearing ring and bearing supports arranged between said bearing rings, and a vibration element for damping, arranged in abutment on the outer bearing ring and the shaft housing and respectively the rotor shaft, said vibration element further serving for heat dissipation, wherein said vibration element has a thermal conductivity of at least 0.3 W/mK.
 2. The pump bearing arrangement according to claim 1, wherein the thermal conductivity of the vibration elements is higher than 0.35 W/mK, preferably higher than 0.4 W/mK.
 3. The pump bearing arrangement according to claim 1, wherein the vibration element comprises an elastomeric material.
 4. The pump bearing arrangement according to claim 1, wherein the vibration element comprises silicone.
 5. The pump bearing arrangement according to claim 1, wherein the vibration element comprises Elastosil, preferably Elastosil R840.
 6. The pump bearing arrangement according to claim 1, wherein a gap is formed between the outer bearing ring and the shaft housing.
 7. The pump bearing arrangement according to claim 1, wherein the vibration element is free of metal particles.
 8. The pump bearing arrangement according to claim 1, wherein the vibration element has a Shore A hardness of 40-85 Shore A.
 9. The pump bearing arrangement according to claim 1, wherein the vibration element has an annular shape.
 10. The pump bearing arrangement according to claim 1, wherein the vibration element has a rectangular or circular cross section.
 11. The pump bearing arrangement according to claim 1, wherein, for damping radial movements, the vibration element surrounds the outer bearing ring.
 12. The pump bearing arrangement according to claim 1, wherein the vibration element is arranged in a groove provided in the shaft housing.
 13. The pump bearing arrangement according to claim 1, wherein a plurality of vibration elements are provided.
 14. Use of a vibration element in pump bearing arrangements, particularly for rapidly rotating pumps, said vibration element having a thermal conductivity of at least 0.3 W/mK.
 15. Use according to claim 14, wherein the vibration element is provided according to claim
 1. 16. The pump bearing arrangement according to claim 1, wherein the thermal conductivity of the vibration elements is higher than 0.4 W/mK, preferably higher than 0.4 W/mK.
 17. A turbomolecular pump comprising: a pump rotor; a rotor shaft connected to the pump rotor; a shaft housing; at least one bearing arrangement which supports the rotor shaft in the shaft housing, the bearing arrangement including: an outer bearing ring, an inner bearing ring connected with the rotor shaft, rolling elements between the inner and outer bearing rings, an elastomeric vibration damping element having a thermal conductivity of at least 0.3 W/mK disposed between the outer bearing ring and the shaft housing. 